Analog-to-digital (A/D) converters operate to produce a numerical representation of an input voltage. One class of A/D converters is known as "dual-slope" A/D converters. Typically, a dual-slope A/D converter integrates the input voltage from a predetermined starting voltage level to a peak voltage level over a predetermined time period, and deintegrates a reference voltage from the peak voltage level back to the starting voltage level. Conversely, a dual-slope A/D converter may integrate a reference voltage over a predetermined time period and deintegrate the input voltage. The time required for deintegration is compared to the predetermined time period to determine the ratio of the reference voltage to the input voltage.
Dual-slope analog-to-digital converters are sold in high volumes for use in digital panel meters, voltage meters, and other instrumentation and control applications. Since they can be readily implemented in either bipolar or MOS technology and are capable of resolution well beyond fifteen bits, they have become the industry's standard for a low-speed, high precision A/D conversion. For maximum resolution and reliability, it is desirable to set the peak voltage level corresponding to the maximum allowable input voltage to an optimum level. Unfortunately, previously developed dual-slope A/D converters require a precision resistor and capacitor which cannot be accurately implemented onto an integrated circuit along with the rest of the A/D circuitry.
Variations in the resistive and capacitive values affect the rate of integration, and hence, adversely affect the peak voltage level obtained during integration. Using an external resistor and capacitor also presents several problems including additional costs, size, parasitic leakage, noise injection and temperature-related variations. These problems reduce the ability of dual-slope A/D converters to compete with "successive approximation" A/D converters in low resolution (less than ten bits) applications and result in high system costs and poorer performance in high precision applications where dual-slope A/D converters are the only available choice.
Therefore, a need exists in the industry for a dual-slope A/D converter which can be integrated on single chip without reduction in accuracy due to the inherent variance in resistor and capacitor values caused by the manufacturing process.